Quay 3.7.0 guide need to add support "Accept Unsigned Images" for image mirror

Description:

This is a doc issue of quay 3.7.0 guide, now in Quay 3.7.0 mirror add support to accept unsigned image "Accept Unsigned Images", but this is not mentioned in the doc and also need to update the related screenshots used in the guide.

Docs: https://quay.github.io/quay-docs/master/manage_quay/index.html#repo-mirroring-in-red-hat-quay